The present invention relates to a method for controlling the conduction cutoff phase angles of p rectifier elements, or thyristors, of a thyristor bridge circuit by forming, in an integrator, for each commutation element group, a signal proportional to the amplitude of the conduction cutoff phase angle by integration of an auxilliary signal of constant amplitude.
Such a method is carried out with the aid of starting pulses derived from decay of the current in the individual rectifiers to zero and end pulses derived at the instant the blocking voltage for a respective rectifier becomes positioned. Integration is separately performed for each element, and the integration result associated with one element is erased by the end pulse associated with another commutation element group, the maximum value of the integrations being stored. The present invention also relates to an apparatus for practicing the method.
A method of the above-described type is disclosed in German Pat. No. 1,263,623, and counterpart U.S. Pat. No. 3,413,544 issued to Jotten et al. This method is used to measure the length of a plurality of successive periodic time intervals by forming a signal representing the average of the successive time intervals. These time intervals correspond to the current flow duration of the p rectifier elements.
If the multiphase, generally three-phase, mains to which a rectifier is connected have a high degree of asymmetry and the mains voltage contains many harmonics, there no longer exists a dependable cutoff angle control for the rectifier because use of an average value precludes rapid follow-up regulation, or adjustment of the regulation upon a reduction of the smallest cutoff phase angle occurs too late.